1. Field of the Invention
The present invention relates to a communication terminal, and more particularly to a communication terminal which uses a key shared across a telecommunications network for encryption and authentication ensuring security of communications. The present invention also relates to a telecommunications system including such a communication terminal.
2. Description of the Background Art
Mesh networks are a kind of telecommunications network including two or more communication terminals which are interconnected in the form of mesh to communicate with each other. On a mesh network, each communication terminal may communicate with other communication terminals, such as adjacent terminals, existing within a range with which the terminal can communicate by its own communication capability. That communication terminal, which may simply be referred to as subject terminal, may communicate with still other communication terminals, such as nonadjacent terminals, existing beyond that range on a multi-hop communication, in which information transmitted from the subject terminal is transferred via an communication terminal adjacent thereto toward a nonadjacent terminal. That means that each communication terminal on a mesh network suffices to emit radio waves having its power to be able to communicate with its adjacent terminals. Furthermore, in a mesh network, if a communication terminal is damaged or disconnected from the network, it is easy to secure an alternative path. Therefore, the mesh network is advantageous particularly in that it is more fault-tolerant than a non-mesh network, such as a conventional star network, which may be rendered incommunicable if a communication terminal serving as a hub suffers from a fault.
A system will now be discussed in which keys are shared across the whole network to encrypt and authenticate communications. In a telecommunications system where security is ensured by providing a shared network key within a network, communication terminals connected to the network have a shared network key. That provides an advantage that the communication terminals connected to the network can perform communications with security maintained without identifying in advance a communication terminal to communicate.
However, under the circumstances where a large number of nodes encrypt and authenticate communication data, it may be undesirable from the viewpoint of security assurance to continuously use the same network key without updating. For example, there is a possibility that an attacker can identify the network key without requiring much time in analyzing such numerous ciphertext messages flowing over the network.
Especially, where quite a lot of nodes are connected to a network, the more frequently the network key is used, the more attacks intend to analyze the network key. For this reason, it is desirable that the network key be periodically updated.
U.S. patent application publication No. 2002/0141591 A1 to Hawkes et al., teaches secure transmissions implemented by each telecommunications node connected to a network having a first broadcast key used for a long time and a second broadcast key used for a short time. The second broadcast key used for a short time is encrypted using the first broadcast key used for a long time, and provided to each node at regular intervals. Broadcast messages sent by a node to all other nodes connected to the network are encrypted using the second broadcast key utilized briefly, and decrypted also using the second broadcast key.
However, where Hawkes et al., is applied to a large-scale network having a large number of telecommunications nodes, the following problems may be raised.
The first problem may be raised when a network key used for encryption and authentication of communications is frequently updated. For example, whenever the network key is updated, a notice of the update is given to all the communication terminals and, therefore, such broadcast traffic frequently occurs in a large-scale network. This places heavy load on communications performed over the whole network.
The second problem is that when the scale of the network is large, it may take a long time for the updated network key to arrive at all the terminals. For example, when some terminals are asleep in power saving mode or a communication path fails, the updated network key does not always arrive instantly at all the communication terminals. In this case, it is impossible to synchronize the communication terminals with each other on the entire network with respect to the network key.